Typically, a fuel battery has a cell stack formed by a number of power generation cells stacked together. With reference to FIGS. 7 to 10, a prior art power generation cell will be described. As shown in FIG. 7, a pair of frames 13, 14 are connected to each other, and an electrode structure 15 is installed at the joint portion of the frames 13, 14. The electrode structure 15 is formed by a solid electrolyte membrane 16, an electrode catalyst layer 17 on the anode side, and an electrode catalyst layer 18 on the cathode side. The outer periphery of the solid electrolyte membrane 16 is held between the frames 13, 14. The anode-side electrode catalyst layer 17 is laid on the upper surface of the electrolyte membrane 16, and the cathode-side electrode catalyst layer 18 is laid on the lower surface of the electrolyte membrane 16. An anode-side gas diffusion layer 19 is laid on the upper surface of the electrode catalyst layer 17, and a cathode-side gas diffusion layer 20 is laid on the lower surface of the electrode catalyst layer 18. Further, an anode-side gas passage forming member 21 is laid on the upper surface of the gas diffusion layer 19, and a cathode-side gas passage forming member 22 is laid on the lower surface of the gas diffusion layer 20. A flat plate-like separator 23 is joined to the upper surface of the gas passage forming member 21, and a flat plate-like separator 24 is joined to the lower surface of the gas passage forming member 22.
The solid electrolyte membrane 16 is formed of a fluoropolymer film. As shown in FIG. 8, the electrode catalyst layers 17, 18 each have granular carbon particles 51 supporting a platinum catalyst, and a great number of platinum (Pt) catalyst particles 52 adhere to the surface of each carbon particle 51. The electrode catalyst layers 17, 18 are bonded to the solid electrolyte membrane 16 by paste for forming an electrode catalyst layer. The catalyst particles 52 serving as a catalyst increase the power generation efficiency when power is generated by the fuel battery. The gas diffusion layers 19, 20 are formed of carbon paper. As shown in FIG. 9, the gas passage forming member 21 (22) is formed of a metal lath, which has a great number of hexagonal ring portions 21a (22a) arranged alternately. Each ring portion 21a (22a) has a through hole 21b (22b). Fuel gas (oxidation gas) flows through gas passages formed by the ring portions 21a (22a) and the through holes 21b (22b). FIG. 9 is an enlarged view showing a part of the gas passage forming member 21, 22.
As shown in FIG. 7, the frames 13, 14 form a supply passage M1 and a discharging passage M2 for fuel gas. The fuel gas supply passage M1 is used for supplying hydrogen gas, which serves as fuel gas, to the gas passages of the anode-side gas passage forming member 21. The fuel gas discharging passage M2 is used for discharging fuel gas that has passed through the gas passages of the gas passage forming member 21, or fuel off-gas, to the outside. Also, the frames 13, 14 form a supply passage and a discharging passage for oxidation gas. The oxidation gas supply passage is located at a position corresponding to the back side of the sheet of FIG. 7, and is used for supplying air serving as oxidation gas to the gas passages of the cathode-side gas passage forming member 22. The oxidation gas discharging passage is located at a position corresponding to the front side of the sheet of FIG. 7, and is used for discharging oxidation gas that has passed through the gas passages of the gas passage forming member 22, or oxidation off-gas, to the outside.
Hydrogen gas from a hydrogen gas supply source (not shown) is supplied to the gas passage forming member 21 through the fuel gas supply passage M1 along a gas flow direction P indicated by an arrow in FIG. 7, and air is supplied to the gas passage forming member 22 from an air supply source (not shown). Accordingly, power is generated through an electrochemical reaction in the power generation cell. Specifically, hydrogen gas (H2) supplied to the anode-side gas passage forming member 21 flows into the electrode catalyst layer 17 through the gas diffusion layer 19. In the electrode catalyst layer 17, hydrogen (H2) is broken down into hydrogen ions (H+) and electrons (e−) as shown by chemical formula (1), and the potential of the electrode catalyst layer 17 becomes zero volts, or the standard electrode potential, as known in the art.H2→2H++2e−  (1)
Hydrogen ions (H+) obtained through the above reaction reaches the cathode-side electrode catalyst layer 18 from the anode-side electrode catalyst layer 17 through the solid electrolyte membrane 16. Oxygen (O2) in the air supplied to the electrode catalyst layer 18 from the gas passage forming member 22 chemically reacts with the hydrogen ions (H+) and the electrons (e−), which generates water as shown by the formula (2). Through the chemical reaction, the potential of the electrode catalyst layer 18 becomes approximately 1.0 volt, or the standard electrode potential, as known in the art.½·O2+2H++2e−→H2O  (2)
Under normal power generation conditions for the fuel battery, the potential of the anode-side electrode catalyst layer 17 (the gas diffusion layer 19) is lower than the potential of the cathode-side electrode catalyst layer 18 (the gas diffusion layer 20), as shown in FIG. 10. Thus, compared to the cathode-side gas passage forming member 22, the anode-side gas passage forming member 21 is less susceptible to metallic oxidation due to a high potential. Therefore, as shown in FIG. 10, an inexpensive stainless steel such as ferrite-based SUS having a low corrosion resistance is used as the material of the gas passage forming member 21. In contrast, the cathode-side gas passage forming member 22, the potential of which can become high, is formed by a metal having a high corrosion resistance such as gold as shown in FIG. 10. Patent Document 1 discloses a fuel battery having a similar structure to the structure shown in FIG. 7.    Patent Document 1: Japanese Laid-Open Patent Publication No. 2007-87768    Patent Document 2: Japanese Laid-Open Patent Publication No. 2007-311089